1. Field of the Invention
The present invention relates to a solid electrolytic capacitor and a method for manufacturing the same.
2. Description of Related Art
With the development of high-performance portable appliances such as mobile telephones and portable music players, capacitors with a smaller size and a higher capacity have been demanded by the market.
As such small-sized and high-capacity capacitor, a solid electrolytic capacitor using a valve metal, such as tantalum (Ta) or aluminum (Al), has been a focus of attention. The solid electrolytic capacitor of this type is manufactured in the following manner. Firstly, a binder formed of a resin and an organic solvent is added to and mixed and kneaded with fine powder of a metal, such as tantalum, or an alloy of the metal. Then, one end of an anode lead is buried in the mixture. The mixture with the buried anode lead is pressed and heated at a high temperature in vacuo to be sintered. Thereby, an anode body formed of a sintered body is formed. Subsequently, the obtained sintered body (anode body) is anodized to form a dielectric oxide film on a surface of the sintered body. Thereafter, a solid electrolyte layer formed of manganese dioxide or a conducting polymer, such as polypyrrole and polyaniline, is formed. Furthermore, a graphite layer and a silver layer are sequentially formed on the solid electrolyte layer. Thereby, a capacitor element is completed.
A method for increasing the capacity of such solid electrolytic capacitor is to increase a surface area of the sintered body to be used as an anode body. A method for increasing the surface area of the sintered body is to reduce the particle size of the metal or the alloy of the metal to be used for sintering.
However, when the particle size of the metal or the alloy of the metal to be used for sintering is reduced, it becomes difficult to form the conducting polymer film, which is formed on the surface of the anode body and is a porous body, in a deep portion inside the anode body. Accordingly, a cathode cannot be formed in a deep portion inside the anode body, and thus a capacity occurrence rate is decreased. As a result, a required capacity cannot be obtained.
To solve the above-described problem, Japanese Patent Application Laid-Open Publication No. Hei 11-274008 discloses a method in which depressurization is carried out when an anode body is soaked in a solution for forming a conducting polymer film.
However, such method has a problem in that the manufacturing process becomes complicated because an anode body has to be disposed in a depressurization chamber when the anode body is soaked in the solution. In addition, even if such method is used, the capacity occurrence rate cannot be sufficiently increased. Accordingly a high capacity cannot be obtained.
Japanese Patent Application Laid-Open Publication No. 2003-136142 discloses a pipe-shaped metal body capacitor and a method for manufacturing the same.